1. Field of the Invention
This invention relates generally to a stereophonic sound reproduction system, and more particularly to an improved stereophonic sound reproduction system which utilizes two loudspeakers.
2. Description of the Prior Art
In a prior art stereophonic sound reproduction system, it has been assumed that the location of the sound image of two channel stereophonic sound signals is limited to the range or region between the two loudspeakers and it has been assumed to be impossible to localize the sound image at positions outside the speakers. For this reason, a so-called quadraphonic system has been widely employed in which, for example, four speakers are located about a listener to reproduce a sound about the listener. This quadraphonic system, however, requires four audio-amplifiers and four loudspeakers, so that it becomes expensive.
It is said that a human listener can discriminate the direction of a sound arriving from his front or left and right relatively correctly, but it is difficult to discriminate the direction of a sound from in back of the listener.
Further, it has been also known to employ a so-called matrix sound system in two-channel stereophonic sound reproduction, in which a signal consisting of a stereophonic left signal combined with a small amount of a stereophonic right signal is supplied to a left speaker, and a signal consisting of the stereophonic right signal combined with a small amount of the stereophonic left signal is supplied to a right speaker, whereby the reproduced sounds from the left and right speakers are matrixed in a reproduction sound field so as to cause a listener to hear the sound as if it originated from positions outside of the two speakers. However, the sound reproduced by this system is rather unclear and hence a natural sounding localization can not be obtained by this system.
In general, in a two-channel stereophonic system, a technique is employed by which the sounds generated from two speakers are composed in a space to appear to be a sound which is emitted from just a single sound source. By way of example, when a sound is desired to be localized at the left-front of a listener, a speaker at the left side is supplied with a signal of relatively high level and a speaker at the right side is supplied with a signal of relatively low level. As described above, the localization of the sound reproduced in ordinary stereophonic sound reproduction systems depends upon the level difference between signals supplied to left and right speakers.
In fact, however, in nature, the direction of a sound generated from a single sound source and the distance between the sound source and a listener are sensed by the listener based upon the ratio between the complex sound pressures produced on the listener's left and right eardrums or the ratio between the absolute values of the sound pressures and also upon the difference between the phases of sounds. Accordingly, even if a single sound is recorded by, for example, a tape recorder and the recorded informations or sounds are reproduced through two speakers, the ratio between the sound pressures and the phase difference between the sounds produced on the eardrums of a listener are different from those experienced when the listener hears the original sound in nature and hence correct localization of the sound cannot be realized.